The present invention relates generally to high performance laser sources internally using nonlinear optical processes to convert laser light to greater frequencies. Merely by way of example, embodiments of the present invention provide an apparatus useful for generating coherent pulsed ultraviolet (UV) light with wavelengths that range between 190 and 350 nm, pulse durations that range between 1 ps and 300 ps, and 10 million or more pulses per second. As another example, embodiments of the present invention provides an apparatus useful for generating pulses of durations lying between 300 ps and 100 ns and occurring 100 thousand to 5 million times each second. In yet another example, pulse packets of a plurality of shorter pulses, each packet having a duration between 1 ns and 1 μs with 100 thousand to 100 million packets occurring each second. However, the scope of the present invention is not limited to this particular implementation and has a broader range of applicability.
Ultraviolet lasers that emit light in the wavelength range 190-350 nm are in increasing demand for many industrial and research applications. UV sources have greater resolving power than visible or near-infrared (NIR) sources, so they are desired for applications requiring high-resolution imaging or scattering. UV sources also have greater energy per photon than visible or NIR sources, and thus they enable many applications that necessitate removal or modification of materials.
Aside from exciplex lasers, in which UV light is directly produced from electronic transitions, ultraviolet lasers use frequency conversion (FC) of at least one lower frequency source laser. Frequency conversion processes include second harmonic generation (SHG), sum frequency generation (SFG), and difference frequency generation (DFG). Frequency-converted lasers contain one or more source lasers, the outputs of which are directed into one or more FC stages of SHG or SFG. These stages are often cascaded in a serial fashion to obtain sequentially higher frequencies. SHG stages convert a portion of the input light into light at a frequency of twice the input light (a wavelength of half of the input light). SFG stages take input light at two different frequencies and convert some of this input light into light at a frequency that is the sum of the input frequencies.
The SHG and SFG FC stages are implemented by using special nonlinear optical (NLO) materials that generate higher frequency light by processes that are now satisfactorily understood and that are categorized under the topic “nonlinear optics.” In many cases the NLO device is a single crystal of a nonlinear optical material that has been precisely cut and oriented to operate for a select purpose. Examples of NLO devices which are more complicated than a single crystal include periodically poled quasi-phase-matching (QPM) devices, devices with walk-off compensation, and the optical contacting of two NLO materials together to implement two FC stages in a single optical component. Great care is taken in order for NLO devices to meet the phase-matching condition, a physical requirement relating to material orientation, orientation of the input beam(s), polarization of the input beam(s), and material temperature. A number of strategies for phase-matching are known in the art, including critical phase-matching, non-critical phase-matching (NCPM), and quasi-phase-matching.
The energy efficiency at which a SHG stage converts light to the second harmonic (SH) increases greatly when the time-averaged input power is delivered in short pulses of high peak power rather than as a continuous wave (CW) of constant magnitude. In fact, when the stage is operating in the regime of low conversion efficiency (below 10%), as is often the case due to various constraints, the conversion efficiency at fixed time-averaged input power is approximately inversely proportional to the duty cycle of the pulsed source. Therefore, it is greatly advantageous to use pulsed sources instead of CW sources in frequency-converted laser systems. In practice, FC laser systems that use CW sources require the use of cavity resonators to enhance the input field strength by constructive interference of circulating laser radiation. While intracavity insertion of a FC device can be made sufficiently robust within a laser gain cavity, external Fabry-Perot external resonators are extremely sensitive to alignment and perturbations. Such cavities pose significant engineering challenges, requiring fast opto-mechanical control and extreme measures to fight thermal and vibrational noise. The engineering is extraordinarily difficult when consecutive resonant cavities are coupled to accomplish higher order optical harmonics.
An ideal solution for many applications that nominally require a CW UV laser is to use rapidly pulsed sources, known as quasi-continuous wave (QCW) sources, in a frequency-converted laser system. The pulsed output signal of such a laser system will appear as a “continuous wave” as long as the repetition rate is higher than the response frequency of the physical system in which it is used. QCW frequency-converted laser apparatuses have the advantage of high source peak power, and thus better internal frequency conversion, while having a high enough repetition rate to appear “CW” in the applications in which they are used.
Typical QCW laser systems use mode-locking techniques. Mode-locking creates a coherent pulse by using constructive interference of the phase-locked laser cavity modes. A major advantage of mode-locking is the ability to maintain a stable output over time. The pulse width will determine the peak power at a fixed repetition rate predefined by the cavity length. A semiconductor saturable mirror (SESAM) is an example of a passive device used for mode-locked lasers which provides short pulse lengths. These devices are self-starting mode-locking components. Other technologies for mode-locking include active types such as frequency modulators (electro-optic effect), amplitude modulators (acousto-optic effect), and passive Kerr lens mode-locking. Pulse lengths produced are typically less than tens of picoseconds (ps) and greater than hundreds of femtoseconds (fs). This pulse length and repetition rate will give the desired characteristics for a QCW source.
An example of a laser system described in U.S. Pat. No. 7,088,744 consists of a mode-locked diode-pumped solid state (DPSS) laser source that uses a conventional laser gain crystal in a Fabry-Perot optical cavity. A SESAM is employed intracavity to modulate gain. An optional frequency converter may be installed intracavity as well, converting a 1064 nm fundamental wavelength to 532 nm by using a NCPM lithium triborate (LBO) crystal. The output 532 nm laser beam can then be used as a source for another FC stage that converts to a wavelength of 266 nm. This entire process forms a mode-locked UV QCW laser system.
Frequency conversion generally benefits from both high peak powers and tightly focused input beams, both of which increase the intensity of the input and output beams within the nonlinear optical material. One of the most important factors that limit the usefulness of frequency-converted UV laser systems is the lifetime of NLO materials under conditions for UV production. Commercial UV NLO devices of prior art are generally fabricated from BBO and CLBO crystals. These NLO devices are unable to support long term, high-output UV light because of their intrinsic weakness in the presence of moisture. Water reacts with the material's surfaces, and penetrates into its bulk, causing breakdown in high intensity laser beams. This failure mode is addressed in some systems by environmental isolation with hermetic cells, elevated temperatures to reduce water sorption, purging dry gasses, and mechanical devices to shift the position of the crystal relative to the laser beam. Every method attempts to overcome the intrinsic faults of BBO and CLBO and resolve longevity issues of the UV NLO process, but with varying degrees of inadequate success. Thus, there is a need in the art for a UV frequency converter that is impervious to water.
A related consequence to the hygroscopic nature of BBO and CLBO: these NLO materials are limited in the degree to which they are able to support high intensity radiation. With activation energy supplied by high intensity input beams, surface damage on the polished faces is quickly promoted in the presence of water. The degradation propagates along the beam path into the bulk device, driven by the high intensity laser beam. This phenomenon limits the amount and duration of input laser radiation through the frequency converter. As a result, conversion efficiencies remain well below optimum and device operational lifetimes are significantly compromised.
It has also been demonstrated that FC efficiency in NLO devices to generate UV light reaches a threshold where increased input intensity does not produce the expected UV output energy. This condition is accounted for by a phenomenon of thermal dephasing where optical absorption within the NLO device leads to localized heating; this causes the refractive indices of the material to change with changing temperature and thereby disrupts optimal phase-matching conditions within the device. Thus, there is a need in the art for methods to reduce thermal dephasing.